Direct Observation of Reverse Magnetic Domain and Magnetic Domain Wall Motion in Nd-Fe-B Magnet at High Temperature by L
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Direct Observation of Reverse Magnetic Domain and Magnetic Domain Wall Motion in Nd-Fe-B Magnet at High Temperature by Lorentz Microscop Toshimasa Suzuki1, Koichi Kawahara1, Masaya Suzuki1, Kenta Takagi2 and Kimihiro Ozaki2 1 Materials Research and Development Laboratory, Japan Fine Ceramics Center (JFCC), Nagoya, 456-8587, Japan. 2 Inorganic Functional Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Nagoya, 463-8560, Japan. ABSTRACT We conducted the in-situ observations of the magnetic domain structure change in NdFe-B magnets at high temperature by transmission electron microscopy (TEM) / Lorentz microscopy with applying an external magnetic field. Prior to observation, a thin foil was magnetized by an external magnetic field of 2.0 T to almost saturation, then the magnetic domain structures were observed by the Fresnel mode with in-situ heating. At 225oC, reverse magnetic domains were found to generate in the thin foil sample without applying an external magnetic field. When we applied a magnetic field on the same direction to the pre-magnetization direction at 225oC, one magnetic domain wall was pinned by a grain boundary and the other magnetic domain wall moved. As the results, the reverse magnetic domain shrank then annihilated. When we cut the applied magnetic field, the reverse magnetic domain generated at almost the same location. On the other hand, when we applied a magnetic field to the foils in the opposite direction, the reverse domain started to grow, i.e., magnetic domain walls started to move. The observation results of the shrink or growth of the reverse domain showed that the pinning effect of grain boundary against domain wall motion would be different depending on the applied magnetic field direction. Moreover, domain walls was observed to be pinned by grain boundaries at elevated temperature, so that the coercivity of Nd-Fe-B magnet would occur by pinning mechanism. INTRODUCTION Nd-Fe-B magnets which have excellent hard magnetic properties have extended the applications in the field of motors for hybrid electric and electric vehicles. The permanent magnet motors are often required to be used at elevated temperature as high as 200oC. It is well known that the Nd-Fe-B magnets without dysprosium (Dy) show the decrease in coercivity at elevated temperatures. In order to realize high coercivity of sintered Nd-Fe-B magnets even at high temperature, various methods have been employed [1-3]. However, the coercivity of developed Nd-Fe-B magnets has not reached to the coercivity of Nd-Fe-B with Dy [4]. Therefore, in order to reduce deterioration of coercivity of Nd-Fe-B magnets at elevated temperatures, it is important to observe generating sites of reverse magnetic domains and pinning sites against magnetic domain walls motion in Nd-Fe-B magnets. The observations of magnetic domain structure change in Nd-Fe-B magnets at room temperature by transmission electron microscopy (TEM) have been reported in refs. 5 and 6. However, since the usage environment
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